The United States Government has rights in this invention pursuant the employer-employee relationship of the Government to the inventors as U.S. Department of Energy employees at the National Energy Technology Laboratory.
The solid oxide fuel cell (SOFC) described here offers an improved approach to the management of waste heat within the fuel cell while reducing the size of heat exchangers and air compressors, thus decreasing the overall space requirement for SOFCs. The fluids circulating within the SOFC are internally heat exchanged and may be controlled to maintain an optimum operating temperature with the SOFC. The internal heat exchange reduces the amount of fluids (air and fuel) required to maintain as uniform a temperature as possible.
Planar SOFCs offer significant advantages for distributed generation applications, particularly for applications which exploit the fuel cell""s potential for reforming fossil fuels such as natural gas and distillates at high power density. However, a number of barrier issues need to be overcome. One of these issues is high temperature heat management. Constrained to operate in the adiabatic mode where heat may not be exchanged with the surroundings, large air-fuel ratios are required in order to maintain the temperature within acceptable operating limits and to control internal thermal stresses, resulting in significant penalties in terms of increased heat exchanger and prime mover duties. The problem is exacerbated by the difficulty of controlling internal reformation of fuel, which would otherwise go a long way to reducing waste heat management loads. After reviewing the commonalities among other types of fuel cells which are nearing commercialization, including tubular solid oxide fuel cells, and internally reforming molten carbonate fuel cells, the principles presented here emerged which are advantageous for planar SOFCs. The concept, internal recuperation, is essentially a division of both anode and cathode chambers into a preheat pass and a reactive pass. Internal recuperation refers to the process by which the heat generated along the reactive pass is given up to the preheat pass. In the cathode chamber, air is counter-currently preheated against the returning air stream as it reacts over the cathode, while in the anode chamber, fuel is both preheated and reformed against the returning fuel stream as it reacts over the anode. Convective and radiative heat transfer coefficients were found to be easily large enough for the anticipated current densities of solid oxide fuel cells. When properly implemented, internal recuperation results in uniform temperature distributions over the electrolyte and related solid structures of the fuel cell. Most importantly, solid structures are shielded from extremes in both low and high temperatures that are deleterious to either operation or structural integrity.
There are other approaches to temperature control involving the bifurcation of fluid flow. One example is in U.S. Pat. No. 5,900,329 (xe2x80x9c""329xe2x80x9d). The ""329 patent describes and claims a dual fuel cell comprising a molten carbonate fuel cell and a solid electrolyte fuel cell where the cathode waste gas is separated into two sub-flows. One sub-flow is supplemented with air, heated by a heat exchanger and sent back to the cathode section of the fuel cell. The fuel cell described in this application is a SOFC and does not require a separate heat exchanger because the fluids are preheated or cooled internally.
An object of this invention is to provide a solid oxide fuel cell and method of use that internally preheats both fuel and air in order to maintain the optimum operating temperature for the production of energy by the addition of a cathode separator plate and an anode separator plate located on either side of the bipolar plate such that these plates create additional passes through the fuel cell.
Another object of the invention is to reduce the amount of fluids (air and fuel) required to maintain an acceptable operating temperature, and thus minimize the space required for power plant construction.
Another object of the invention is to eliminate the requirement of an external heat exchanger unit by use of the described SOFC or method and, thus minimize the size of equipment and conduits required for power plant construction and operation.
Another object of the invention is to allow the addition of air or fuel to the fuel cell as required to maintain the optimum operating temperature through a cathode control valve or an anode control valve, respectively. The control valves are part of a control loop that comprises a temperature sensing means within the preheat air and fuel passes, a means to compare the measured temperature to a set point temperature and a determination based on the comparison as to whether the control valves should allow additional air or fuel into the fuel cell.
The invention is a solid oxide fuel cell arrangement and method of use that provides internal preheating of both fuel and air in order to maintain the optimum operating temperature for the production of energy. The internal preheat passes are accomplished by the addition of a cathode separator plate and an anode separator plate located on either side of the bipolar plate such that these plates create additional passes through the fuel cell. This internal preheat fuel cell configuration and method eliminate the requirement of an external heat exchanger unit. Air or fuel may be added to the fuel cell as required to maintain the optimum operating temperature through a cathode control valve or an anode control valve, respectively. The control valves are part of a control loop that comprises a temperature sensing means within the preheat air and fuel passes, a means to compare the measured temperature to a set point temperature and a determination based on the comparison as to whether the control valves should allow additional air or fuel into the fuel cell.